Gödel's incompleteness theorems first appeared together in a paper titled (translated to English) "On formally undecidable propositions of Principia Mathematica and related systems I," with the Roman numeral at the end of the title indicating that this was to be the first paper of a series to be published. The only explicit mention of subsequent papers I've found is in Torkel Franzén's book on the incompleteness theorems which mentions (only in passing) that a second paper was planned but never written.
My question is: what was the planned subject of this second paper? I'm intrigued by the thought that such fundamental work was only the first half of a planned corpus of research. In addition, if someone happens to know why the paper was never written, I'd be interested in learning about why.